Single Sideband modulation (SSB) is very efficient in the use of the frequency spectrum. Other common modulations, such as amplitude modulation (AM) and frequency modulation (FM), are very inefficient. AM takes twice as much spectrum and FM can take 4 to 8 times the spectrum. Since frequency spectrum is a scarce resource, any technology that can conserve frequency spectrum is of high value.
SSB is also very power-efficient. Compared to AM, SSB communications can be made with less than one tenth the power. Reducing the transmitted power reduces the interference to other communication services and thereby also improves the frequency spectrum usage.
However, SSB signals need to be tuned within approximately 10 Hertz (Hz) to avoid significant audio distortion. Signals mistuned much beyond this limit sound either like a deep rumble or like Donald Duck, depending on the direction of mistuning.
One solution has been to transmit only on certain specific frequencies (channels). However, this requires that both the high-frequency transmitter and receiver be tuned to exactly the correct frequency. This may require tuning to within about 20 parts per billion, depending on the carrier frequency. This degree of accuracy is expensive to implement, particularly over a wide range of environmental conditions and must be maintained over the expected lifetime of the radio. This is the reason that Marine HF SSB radios have a “clarifier” control for operator adjustment of the receiver frequency. This adjustment is somewhat difficult to use and requires practice to adjust for adequate audio quality. A second disadvantage of channelized operation is reduced spectral efficiency. It is often advantageous to slightly change frequency to avoid RF interference instead of abandoning the channel altogether and shifting to another channel.
A second solution, well-known to those skilled in the art, is to add a known frequency audio tone (pilot tone) to the transmitted signal. If the receiving station knows the transmitted pilot tone frequency, it can automatically adjust the received frequency to set the received pilot tone to the desired frequency. There are at least three disadvantages to this solution. First, the transmitter and receiver must be designed to work with the same pilot tone frequency and amplitude. This discourages the formation of ad hoc communications and is incompatible with existing radio infrastructure. Considering the large number of SSB transceivers in use today, updating this equipment is impractical and inventions using pilot tones are of limited utility. Second, the added pilot tone needlessly consumes transmitter power. Maximum transmitted power is usually limited by regulation; so wasted power reduces range and the readability of the signal. Finally, receiver bandwidth is limited to minimize noise, and interference. Therefore, if the receiver is mistuned by more than a few hundred Hz, the pilot tone can be filtered off and the automatic tuning will fail.
Several tuning techniques attempt to use the properties of voice signals to automatically tune SSB voice signals (see, for example, “Co-Channel Interference Separation” by Robert Dick, December 1980, “Tune SSB Automatically” by Robert Dick, QEX magazine, January/February 1999, “A Blind Automatic Frequency Control Algorithm for Single Sideband” by Gary Geissinger, QEX magazine July/August 2005, and “Communications Receivers” by Dr. Ulrich Rohde. None of these techniques have successfully and consistently tuned actual voice SSB signals.
In contrast with the above prior art, the invention requires no modifications to the transmitter and so a receiver equipped with this invention can be used with any SSB transmitter in use today. It can also correct for much larger tuning errors. As discussed in detail below, this invention analyzes the properties of the transmitted human voice, independent of language and retunes the receiver to the actual transmitted signal frequency with a high degree of accuracy. This can be done faster than a trained operator can retune the radio.